1. Field of the Invention
This invention relates to antenna control systems and, more particularly, to antenna control systems for pointing and controlling the directional antennas of communications satellites.
2. Description of the Prior Art
To obtain optimum communication coverage over an area being served by a communications satellite, precise directional satellite antenna control is necessary. The complexity of antenna control systems depends upon many factors, including the type of satellite, the type and extent of communication coverage, the satellite orbit, and so on. Notwithstanding the many differences in satellite types and missions, it is necessary to orient and direct the antennas so that their transmit/receive beams coincide with the desired coverage on earth.
Antenna direction control becomes increasingly complex as improved aiming accuracy and longer satellite lifetimes are sought. In one prior art antenna control system for spin-stabilized satellites, redundant earth sensors and sun sensors provide the basic sensing elements, and a processor derives the steering signals for a de-spin motor to control the antenna pointing. Such systems have several drawbacks including cost, complexity and weight. In addition, pointing errors associated with sensor noise and thermal deformation, for example, are present in such systems.
An improved antenna control system is described in U.S. Pat. No. 3,757,336 which issued to the present inventor on Sept. 4, 1973. In that patent, the communications satellite antenna control is provided by means of a pilot signal transmitted from an earth station. This earth station transmits a modulated beacon or pilot signal to the satellite where it is received, processes, decoded and utilized to control the de-spin motor in the satellite for tracking and offset.
As narrower antenna beams are used in communication satellite service as a consequence of the higher frequencies employed, much more precise antenna beam pointing accuracies are required. With these narrower antenna beams it is becoming increasingly necessary to provide both east-west and north-south beam pointing with high accuracies.
It is, therefore, an object of the present invention to provide a low-cost, two-axis satellite antenna pointing and control system.
Another object of the invention is the provision of a satellite antenna pointing and control system which permits very precise beam pointing to be achieved in a simple manner.
Another object of the present invention is to provide a satellite antenna pointing and control system which eliminates potential sources of satellite failure by locating system complexities at the ground station instead of on board the satellite.
Still another object of the invention is the provision of a satellite antenna pointing and control system which makes use of apparatus which must be provided on the satellite anyway for other purposes.
Yet another object of the present invention is to provide a satellite antenna pointing and control system which operates with low noise as well as low calibration errors.
A further object of the invention is the provision of a satellite antenna pointing and control system which permits the ground transmitter to have a low spectral power density.